Epoxy and polyurethane coatings are widely used for protection of substrates against corrosion and erosion caused by use and the environment. The types of surfaces treated include concrete, metal, and other surfaces. In addition to protection of substrates, coating systems need to be cost effective, easily applied and cured, have zero or very low VOC (volatile organic compound) and low toxicity.
Conventional epoxy resin systems have great adhesion to a number of substrates with good anticorrosive properties, lower coat, and relatively low toxicity when compared to isocyanates. However, epoxy coatings typically have poor durability or weathering characteristics when exposed to ultraviolet light. Further, most conventional epoxy systems require a high level of VOC for application.
Conventional polyurethane coatings have much better durability than epoxy resins with a greater resistance to yellowing and loss of gloss. Polyurethanes, utilizing solvents for thinning, have increased toxicity issues due to isocyanates and solvents, higher cost, and environmental issues due to VOC emissions.
Current state of the art methods for protection of metal substrates are to use an epoxy primer with a urethane topcoat to provide ultraviolet light protection for the system. Because of the increased costs of doing a two-coat system, OEM manufactures would prefer a one coat DMT (Direct To Metal) application that would provide both durability and excellent corrosion protection.
The use of epoxy compositions containing volatile organic solvents has fallen out of favor even though these compositions provide exceptional anti-corrosion protection. The organic solvent based compositions are environmentally unfriendly and their use has been curtailed by increasingly stringent regulation. Solvent free epoxy coatings often require sophisticated, non-standard equipment for application. Thus, the coating industry has turned to waterborne systems to address the VOC issue. Waterborne technology, as addressed in the co-pending application referenced hereinabove, has great potential to address VOC issues because water can be used for lowering application viscosity instead of the use of volatile organic compounds. One of the main concerns with conventional waterborne systems and improved in the above referenced co-pending application, has been quality issues for high build one-coat systems. Conventional waterborne coating systems are unable to provide high build one-coat systems because of foaming issues. These issues have been overcome as described herein below as well as the above referenced co-pending application.
Coating compositions provide good protection if applied at the recommended thickness. This recommended thickness requires multiple coats and each coat must be cured or set before another coat may be applied. Each additional coat requires additional labor, which increases the cost of application. Other coats may be applied at greater initial thickness, but require more time to set up. Coatings applied at greater initial thicknesses may also introduce flaws caused by voids from evaporation of volatile components during the curing process.
Conventional waterborne polyurethane coatings must still deal with the toxicity issues of handling isocyanates. In at least one embodiment, the instant disclosure presents a coating system having the ability to deliver a non-isocyanate DTM one-coat waterborne system at a VOC of less than 50 grams per liter at a film thickness of greater than 12 mils with better durability than a conventional epoxy resin.
Until the discoveries disclosed in the related application referenced herein, the coatings industry has been unsuccessful in developing a two part, water-borne polyurethane system that would build film thickness, like a solvent-borne industrial coating, where a minimum film build of two to three mils dry film thickness (DFT) is desired for commercial applications. Film thickness greater than two to three mils wet film thickness (WFT) in a water-borne polyurethane system had resulted in foaming and gassing. The foaming and gassing was primarily due to the reaction of isocyanate with moisture. Further conventional two part, water-borne polyurethane systems cannot achieve the desired product flow during application. The lack of proper flow frequently results with an eggshell type appearance which is unacceptable in higher scale commercial painting. Still further, newer environmental restrictions, that are being implemented across the United States, limit and/or eliminate the use of solvent-borne polyurethanes. Currently, the use of ultraviolet curing and polyasparitic technologies have been used to attempt an acceptable low VOC/HAP's free system.
Initially, aqueous polyurethane dispersions (PUDs), which may be one and/or two-component coating systems, appeared in response to higher solvent prices and the increased demand for low-VOC coatings. These are usually made by reacting mixtures of polyols and dimethylolpropionic acid with a polyisocyanate to give a complete polyurethane or an isocyanate-terminated prepolymer. This product is then dispersed in water (which may contain other isocyanate-reactive compounds) by neutralizing the acid groups with a base, typically a tertiary amine. Aqueous PUDs provide a low, but not zero VOC alternative to conventional two-component, solvent-based coating formulations. However, because they are only lightly crosslinked, coatings from aqueous PUDs often lack adequate solvent resistance, water resistance, gloss, hardness, and weathering properties. In addition, a cosolvent is usually needed for good coalescence, so solvents are not easy to eliminate from the formulations and therefore the mandated environmental requirements of low VOC's and HAPs have been difficult to achieve.
In the early 1990s, two-component (2K) aqueous polyurethane coatings arrived on the scene (see generally: P. Jacobs et al., “Two-Component Waterborne Polyurethane Coatings: Now and Into the Next Century” and cited references). Scientists discovered that it is possible to use water as a carrier for reactive 2K systems and still get coatings with good appearance and physical properties. Two-component aqueous polyurethane coating formulations are typically dispersions of separate polyol and polyisocyanate moieties. A coating film forms after water evaporates and the components react to give a crosslinked polymer network. While 2K aqueous polyurethane coatings should, in theory, match the properties available from solvent-based 2K systems, the coatings have, in practice, lacked adequate water, solvent, and chemical resistance (particularly, but not limited to, resistance to Skydrol), gloss retention, weatherability, flexibility, and impact resistance.
The success of aqueous 2K systems has, until now, relied on some important and often unwieldy formulation twists. For example, the polyol required, which needs both hydroxyl functionality for the polyurethane-forming reaction and acid groups for water dispersibility, is usually not commercially available. In one approach, an acrylate polymer with acid and hydroxyl functionalities is made by copolymerizing (in a free-radical polymerization) an acrylic acid monomer and a hydroxyalkyl acrylate monomer (e.g., hydroxyethyl acrylate or hydroxyethyl methacrylate). Unfortunately, hydroxyalkyl acrylates are rather expensive. In addition, it is difficult to make hydroxyalkyl acrylate polymers that have both high hydroxyl functionality and molecular weights low enough to have value for low-VOC, crosslinkable coating systems. The result is a lower level of coating physical properties than would otherwise be desirable. Recently developed hydroxy-functional acrylate polymers based on allylic alcohols and alkoxylated allylic alcohols overcome some of the limitations of using hydroxyalkyl acrylate monomers. However, the value of these resins has, until now, been demonstrated primarily for solvent-based polyurethane coatings or with high-styrene (>50 wt. %) resins, and not for aqueous polyurethane coatings.
A second common way to tweak the 2K aqueous polyurethane coating formulation is to modify the polyisocyanate. Most of the work to date has used a polyisocyanate modified by partially reacting it with a hydrophilic polyether. Making the polyisocyanate hydrophilic provides an emulsifiable crosslinker having improved compatibility with the co-reactants. This approach also has disadvantages, however. First, the hydrophilic polyisocyanate must be synthesized. Second, more of the expensive hydrophilic polyisocyanate must be used (compared with the unmodified polyisocyanates) to get the same NCO functionality contribution. Third, the hydrophilicity of the polyisocyanate is incorporated into the coating, often making its water sensitivity unacceptably high.
A third approach modifies the processing while keeping a commercial polyisocyanate in the formulation. The key concern is how to adequately disperse the polyisocyanate in water because emulsions made from commercial polyisocyanates tend to agglomerate and settle. Adding cosolvents and emulsifiers can help, but this at least partially defeats the purpose of using an aqueous system.
At present, two-package solvent-based polyurethane coatings are widely used as coatings for substrates, such as metals, wood, and plastics. These two-package solvent-base urethanes can be cured at room temperature or cured at relatively lower temperature. Such urethanes yield coatings with high levels of mar resistance and chemical resistance. They are so good that they often give more protection than is needed by the substrate. Because these coating compositions are made with organic solvents, which must be liberated into the atmosphere, they cause environmental problems which makes desirable a switch to non-toxic, e.g., aqueous-based compositions. Moreover, because the solvent-containing compositions are often reduced (i.e., thinned) with strong non-polar to medium polar solvents, they often attack and degrade plastic substrates to an undesirable degree. Non-polar thermoplastics, such as aromatic polycarbonates, e.g., of bisphenol-A and phosgene, or polyphenylene ethers, e.g., poly (2,6-dimethyl-1,4-phenylene) ethers, are capable of being dissolved and/or attacked by such non-polar solvents, and they can be distorted with excessive heat.